Have I ruined my batteries?

[NapalmFist]

I have a battery bank that hasnt yet been installed in my solar setup. Its 64 cells rated at 304AH each. I ordered these all from docan. I top balanced my entire bank at 3.7 volts with a power supply. Then the resting voltage didnt fall to 3.4 as expected, it stayed just under 3.6 volts and barely dropped over the next 3 weeks. Then I read that I shouldnt store at 100% capacy so i decided to drain some off to get to about 3.34 volts. All I had to do this with was two halogen automotive bulbs that draw less than 50 watts. To drain my entire bank down to 3.34 volts took about 4 hours. This is astonishing to me because that is 10% of my battery capacity and my bank should be over 60 kwh of capcity, so I expected it would take 6000 watts of energy not less than 400 (in fact my multimeter read the bulb as onlybdrawing 2.5 amps so 25 watts each). Can someone explain what is going on, Im loosing my mind of it.

 

[Zwy]

I'd cycle the thing a few times. Most likely the damage is slight, some possible lithium plating but probably not much capacity loss. . As for the drop in voltage to 3.34V, I wouldn't worry about it. It is normal for cell voltage to drop when charging finishes and cells allowed to sit. As long as these are not severely bloated, I would use them.

Let this be a lesson to top balance correctly and not rush the job. I use a BMS to get the initial charge until one cell hits 3.55V (set in the BMS), then finish in parallel if the cells are heading to a passive balancer BMS, charging to 3.60V in parallel. For active balancer BMS's, I will charge until one cell gets to 3.50V to 3.55V, then stop charging and wait until the balancer is done, then keep repeating the process until all cells are at 3.55V to 3.60V. I don't need to get to 3.65V.

 

[Partimewages]

To add don't go to 3.7 volts! 3.65 is the maximum.

 

[AntronX]

This is astonishing to me because that is 10% of my battery capacity and my bank should be over 60 kwh of capcity

You did not draw 10% of your capacity. Above 3.35v at rest is 95 - 100%. Only way to know true capacity for sure it fully charge to 3.6v 0.05C cutoff amps and discharge to 2.5v 0.05C cutoff. I would not worry too much over this. Whats done is done so don't do it again. My guess you lost few % to SEI layer growth that would have grown anyway during first year of normal operation. What temperature were they stored at?

 

[BentleyJ]

The normal resting voltage of a fully charged cell is 3.35V. Yes, the cell voltage will continue to climb if current continues to be forced into the fully charged cell. However, this is what I would call a capacitive effect as there is NO additional chemical storage capacity left. When the halogen bulbs were connected they rapidly drained away the excess which has nothing to do with the actual stored chemical Ah capacity.

The literature I have read suggests that the electrolyte does not start to break down until the cell reaches 4.2V. At 3.7V there was likely minimal damage, if any.

 

[NapalmFist]

You did not draw 10% of your capacity. Above 3.35v at rest is 95 - 100%. Only way to know true capacity for sure it fully charge to 3.6v 0.05C cutoff amps and discharge to 2.5v 0.05C cutoff. I would not worry too much over this. Whats done is done so don't do it again. My guess you lost few % to SEI layer growth that would have grown anyway during first year of normal operation. What temperature were they stored at?

Stored at 20 celcius. The other concern is that it didnt just rapidly drop to 3.34 but if i left it even 5 more minutes it would drop to 3.2 (i did this in groups of in series). I suppose there is nothing I can do but its hard ro imagine they will run anything for a significant lenth of time when you can watch the voltage drop continuously off a lightbulb.

 

[AntronX]

That does not sound right. Do a capacity test.

 

[Dirtywhitehat]

To add don't go to 3.7 volts! 3.65 is the maximum.

I was reading something about that. Because I kinda raised an eyebrow when looking at 48v systems and FULLY charged with each cell at 3.7 volts 16S would be like …59.2 volts

Which is outside the operating parameters of most inverters designed to deal with those systems right?

So something like 3.55 seemed to make more sense?